Marc-Etienne Moret

Terminal Iron Dinitrogen and Iron Imide Complexes Supported by a Tris(phosphino)borane Ligand

Adaptable metallaboratranes: A tris- (phosphino)borane ligand stabilizes both low-valent Fe-N_2 complexes and a mid-valent imido species with a Fe≡NAr bond, thanks to its ability to shuttle between trigonal-bipyramidal and pseudotetrahedral geometries by elongation of the apical iron–boron bond (see picture)

N2 Functionalization at Iron Metallaboratranes

The reactivity of the anionic dinitrogen complex [(TPB)Fe(N(2))](-) (TPB = tris[2-(diisopropylphosphino)phenyl]borane) toward silicon electrophiles has been examined. [(TPB)Fe(N(2))](-) reacts with trimethylsilyl chloride to yield the silyldiazenido complex (TPB)Fe(NNSiMe(3)), which is reduced by Na/Hg in THF to yield the corresponding sodium-bound anion [(TPB)Fe(NNSiMe(3))]Na(THF). The use of 1,2-bis(chlorodimethylsilyl)ethane in the presence of excess Na/Hg results in the disilylation of the bound N(2) molecule to yield the disilylhydrazido(2-) complex (TPB)Fe≡NR (R = 2,2,5,5-tetramethyl-1-aza-2,5-disilacyclopentyl). One of the phosphine arms of TPB in (TPB)Fe≡NR can be substituted by CO or (t)BuNC to yield crystalline adducts (TPB)(L)Fe≡NR (L = CO, (t)BuNC). The N-N bond in (TPB)((t)BuNC)Fe≡NR is cleaved upon standing at room temperature to yield a phosphoraniminato/disilylamido iron(II) complex. The flexibility of the Fe-B linkage is thought to play a key role in these transformations of Fe-bound dinitrogen.

Gas-Phase Synthesis and Reactivity of a Gold Carbene Complex

Electrospray ionization tandem mass spectrometry of a phosphonium ylid complex of gold produces an ion whose mass and gas-phase chemical reactivity indicate that it is a gold benzylidene complex. The complex, with a supporting NHC ligand, corresponds to a type of reactive intermediates which have been presumed to act in gold-catalyzed cyclopropanation reactions, but which have not been observed to date in solution or gas-phase experiments. A threshold CID experiment also yields thermochemical information on the formation of the gold carbene from the ylid complex precursor.

Conversion of Fe-NH2 to Fe-N2 with release of NH3

Tris(phosphine)borane ligated Fe(I) centers featuring N(2)H(4), NH(3), NH(2), and OH ligands are described. Conversion of Fe-NH(2) to Fe-NH(3)(+) by the addition of acid, and subsequent reductive release of NH(3) to generate Fe-N(2), is demonstrated. This sequence models the final steps of proposed Fe-mediated nitrogen fixation pathways. The five-coordinate trigonal bipyramidal complexes described are unusual in that they adopt S = 3/2 ground states and are prepared from a four-coordinate, S = 3/2 trigonal pyramidal precursor.

Interaction of organoplatinum(II) complexes with monovalent coinage metal triflates.

The organoplatinum(II) complexes [(NN)PtMe(2)] and [(NN)PtPh(2)] (NN = ArNC(Me)C(Me)NAr, Ar = 2,6-dichlorophenyl) can act as donor ligands for copper(I) and silver(I) triflates, affording a series of homo- and heteroleptic complexes which were characterized by X-ray diffraction. [(NN)PtMe(2)] binds to the coinage metals through short, ligand-unsupported d-d(10) contacts that are best described as Pt-->M dative bonds (M = Cu, Ag), in which the d(z(2)) orbital of the square-planar Pt(II) center donates electron density to the Lewis-acidic metal. Spectroscopic studies in solution and DFT calculations corroborate this description. [(NN)PtPh(2)] binds preferentially by eta(1) or eta(2) complexation of the ipso carbon atoms of the phenyl groups to the coinage metal, affording homoleptic complexes {[(NN)PtPh(2)](2)M}(+){TfO}(-) in the solid state. The 1:1 adducts of formula {[(NN)PtPh(2)]M(OTf)}(n) (M = Cu, n = 1; M = Ag, n = 2) are observed in solution, and a 1:2 adduct of formula {[(NN)PtPh(2)]Ag(2)(OTf)(2)(C(6)H(6))}(n) was characterized in the solid state, showing that unsupported Pt-->M bonds are also accessible for [(NN)PtPh(2)]. The thermolyses of the complexes [(NN)PtMe(2)]MOTf in benzene affords moderate yields of [(NN)PtPh(2)] through an oxidatively induced double C-H activation process.

A Ru(I) Metalloradical That Catalyzes Nitrene Coupling to Azoarenes from Arylazides

Unusual N-N coupling of aryl azides to yield azoarenes is demonstrated by the Ru(I) metalloradical, [SiP(iPr)(3)]Ru(N(2)) (4) ([SiP(iPr)(3)] = (2-iPr(2)PC(6)H(4))(3)Si(-)). The yield of the azoarene is dependent on the substituent on the aryl azide, and the reaction is catalytic for p-methoxy and p-ethoxy phenyl azides, while no azoarene is observed for p-trifluoromethylphenyl azide. Studies aimed at probing the viability of a bimolecular coupling mechanism of metal imide species, as shown in the related [SiP(iPr)(3)]Fe system, have led to the isolation of several structurally unusual complexes including the ruthenium(IV) imide, 7-OMe, as well as the Ru(II) azide adduct 8-OMe. One electron reduction of 7-OMe complex led to the isolation of the formally Ru(III) imide complex, [SiP(iPr)(3)]Ru(NAr) (Ar = p-MeOC(6)H(4), 5-OMe). EPR spectroscopy on 5-OMe suggests that the complex is electronically similar to the previously reported imide complex, [SiP(iPr)(3)]Ru(NAr) (Ar = p-CF(3)C(6)H(4,)5-CF(3)), and features radical character on the NAr moiety, but to a greater degree. The stability of 5-OMe establishes that bimolecular coupling of 5-OMe is kinetically inconsistent with the reaction. Further studies rule out mechanisms in which 5-OMe reacts directly with free aryl azide or a transient Ru(I) azide adduct. Together, these studies show that 5-OMe is likely uninvolved in the catalytic cycle and demonstrates the influence of the metal center on the mechanism of reaction. Instead, we favor a mechanism in which free aryl nitrene is released during the catalytic cycle and combines with itself or with free aryl azide to yield the azoarene.

Transmetalation Supported by a PtIICuI Bond

Passing Me over: platinum-to-copper methyl transfer is observed upon collision-induced dissociation (CID) of the cations formed by the interaction of the [(R(3)P)Cu](+) fragment with [(dmpe)PtMe(2)] (R=Me, Ph, Cy, tBu; dmpe=bis(dimethylphosphino)ethane; see X-ray crystal structure of coordination spheres for R=tBu). The thermochemistry of these processes for R=Me is investigated by mass-spectrometric methods.

A polar copper-Boron One-Electron σ-bond

Virtually all chemical bonds consist of one or several pairs of electrons shared by two atoms. Examples of σ-bonds made of a single electron delocalized over two neighboring atoms were until recently found only in gas-phase cations such as H2(+) and Li2(+) and in highly unstable species generated in solid matrices. Only in the past decade was bona fide one-electron bonding observed for molecules in fluid solution. Here we report the isolation and structural characterization of a thermally stable compound featuring a Cu-B one-electron bond, as well as its oxidized (nonbonded) and reduced (two-electrons-bonded) congeners. This triad provides an excellent opportunity to study the degree of σ-bonding in a metalloboratrane as a function of electron count.

Transmetalation of Methyl Groups Supported by PtII–AuI Bonds in the Gas Phase, in Silico, and in Solution

We report Pt(II)-to-Au(I) methyl transfer reactions that occur in the gas phase and in solution. The heterobimetallic Pt(II)/Au(I) complexes {[(dmpe)PtMe(2)][AuPR(3)]}(+) (R = Me (2a), Ph (2b), (t)Bu (2c)), observed in the gas phase by means of electrospray ionization, were subjected to collision induced dissociation (CID) from which we could observe Pt-to-Au transmetalation along two reaction pathways involving formation of a Au-Me bond, analogous to those observed for the Pt(II)/Cu(I) complex recently reported. In the first pathway, neutral AuMe is generated with concomitant migration of PR(3) from Au(I) to the Pt(II) center, forming cation [(dmpe)PtMe(PR(3))](+) (R = Me (5a) or Ph (5b)). In the second pathway, the monophosphine stays attached to the gold center, yielding cation [(dmpe)PtMe](+) (7) and R(3)PAuMe. Quantitative energy-resolved collision induced dissociation experiments as well as density functional theory (DFT) calculations were used to investigate the potential surface involved in the transmetalation processes. Energy barriers of 22.3 and 47.9 kcal mol(-1) for the two reaction processes of 2b and of 45.4 kcal mol(-1) for the single reaction process of 2c were obtained. Parallel reactivity is observed in THF solution, allowing for a comparison of the product distributions with those observed in the gas phase, and the postulation of simple steric control of the branching ratio between the two pathways. DFT calculations at the M06-2X//BP86/TZP level were in good agreement with the experiments.

Organometallic Platinum(II) and Palladium(II) Complexes as Donor Ligands for Lewis-Acidic d^(10) and s^2 Centers

Square-planar palladium(II) and platinum(II) complexes with a high-lying filled dz² orbital can act as metalloligands for Lewis-acidic metal centers such as d10 and s2 cations. This behavior is promoted by hard ligands such as σ-bound hydrocarbyl ligands. A wide diversity of structural motifs based on this kind of donor–acceptor metal–metal bonds has been discovered in the last decades. This chapter reviews the coordination chemistry of metalloligands derived from alkyl, aryl, alkynyl, and carbene complexes of palladium(II) and platinum(II). The specific reactivity of the resulting bimetallic complexes is also addressed.